Update master version to 3.8.0

Motivation
----------
The master branch assembly version is still stamped with 3.7.0.
I propose we bump it to 3.8.0 (instead of SNAPSHOT or other which may
cause some issues with jenkins/FIT).

This will help differentiate it to other FIT-SIT/FIT-PERF runs we do on the
v3.7 branch.

Change-Id: I7aa142290e34822dc9f91702cfec3c26a179edd6
Reviewed-on: https://review.couchbase.org/c/couchbase-net-client/+/227539
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Michael Reiche <michael.reiche@couchbase.com>

Author: emilienbev

Directory.Build.props

@@ -20,7 +20,7 @@
   </PropertyGroup>
   <PropertyGroup>
     <!-- update this after a release-->
-    <CouchbaseNetClientReleasedVersion>3.7.0</CouchbaseNetClientReleasedVersion>
+    <CouchbaseNetClientReleasedVersion>3.8.0</CouchbaseNetClientReleasedVersion>
   </PropertyGroup>
   <PropertyGroup>
     <!-- libraries typically include NetStandardTargets for widest use cases -->